The present invention relates to a cutting tool for metal machining, having a substrate of cemented carbide, cermet, ceramics or high speed steel and, on the surface of said substrate, a hard and wear resistant refractory coating is deposited by Physical (PVD) or Chemical (CVD) Vapor Deposition. The coating is adherently bonded to the substrate and is composed of a laminar, multilayered structure of alternating metal nitrides or carbides with individual layers, or lamellae, having an aperiodic sequence of thicknesses. With this structure, it is understood that the multilayer has no superlattice repeat period. Individual metal nitride or carbide layers have thicknesses in the nanometer range (nm) and the metal elements of the nitride or carbide are selected from the group consisting of Ti, Nb, Hf, V, Ta, Mo, Zr, Cr Al, W and mixtures thereof.
The present invention relates particularly to the art of PVD coated cemented carbides or similar hard materials such as cermets, ceramics and high speed steels. The method of depositing a thin refractory coating (1-20 .mu.m) of materials like alumina (Al.sub.2 O.sub.3), titanium carbide (TiC) and/or titanium nitride (TiN) onto, e.g., a cemented carbide cutting tool, is a well-established technology and the tool life of the coated cutting tool, when used in metal machining, is considerably prolonged. The prolonged services life of the tool may, under certain conditions, extend up to several 100 percent. Refractory coatings known in the art comprise either a single layer or a combination of multilayers. Modern commercial cutting tools are characterized by a plurality of layer combinations with double or multilayer structures. The total coating thickness varies between 1 and 20 .mu.m and in the prior art, the multilayered structure is characterized in the micrometer range (.mu.m), i.e., the thickness of the individual sublayers varies between a few microns and a few tenths of a micron.
The established technologies for depositing such coatings are CVD and PVD (see, e.g., U.S. Pat. Nos. 4,619,866 and 4,346,123). PVD coated commercial cutting tools of cemented carbides or high speed steels usually have a single coating of TiN, TiCN, or TiAlN, but combinations thereof also exist.
There exist several PVD techniques capable of producing refractory thin layers on cutting tools. The most established methods are ion plating, magnetron sputtering, arc discharge evaporation and IBAD (Ion Beam Assisted Deposition). Each method has its own merits and the intrinsic properties of the produced coating such as microstructure/grain size, hardness, state of stress, cohesion and adhesion to the underlying substrate may vary depending on the particular PVD method chosen. An improvement in the wear resistance or the edge integrity of a PVD coated cutting tool being used in a specific machining operation can thus be accomplished by optimizing one or several of the above-mentioned properties. Furthermore, new developments of the existing PVD techniques by, for instance, introducing unbalanced magnetrons in reactive sputtering (S. Kadlec, J. Musil and W.-D. Munz in J. Vac. Sci. Techn. A8(3), (1990), 1318) or applying a steered and/or filtered arc in cathodic arc deposition (H. Curtins in Surface and Coatings Technology, 76/77, (1995), 632 and K. Atari et al. in Surface and Coatings Technology, 43/44, (1990), 312) have resulted in a better control of the coating processes and a further improvement of the intrinsic properties of the coating material.
Conventional cutting tool materials like cemented carbides comprise at least one hard metallic compound and a binder, usually cobalt (Co), where the grain size of the hard compound, e.g., tungsten carbide (WC), is in the 1-5 .mu.m range. Recent developments have predicted improved tool properties in wear resistance, impact strength and hot hardness by applying tool materials based on ultrafine microstructures by using nanostructured WC-Co powders as raw materials (L. E. McClandlish, B. H. Kear and B. K. Kim, in NanoSTRUCTURED Materials, Vol. 1, pp. 119-124, 1992). Similar predictions have been made for ceramic tool materials by, for instance, applying silicon nitride/carbide-based (Si.sub.3 N.sub.4 /SiC) nanocomposite ceramics and, for Al.sub.2 O.sub.3 -based ceramics, equivalent nanocomposites based on alumina.
With nanocomposite nitride or carbide hard coating materials, it is understood a multilayered coating where the thickness of each individual nitride (or carbide) layer is in the nanometer range, 3-100 nm, or preferably 3-20 nm. Since a certain periodicity or repeat period of, e.g., a metal nitride layer sequence is invoked, these nanoscaled, multilayer coatings have been given the generic name of "superlattice" layers. A repeat period is the thickness of two adjacent metal nitride layers, i.e., with different metal elements in the sublayers. Several of the metal nitride superlattice coatings with the metal element selected from Ti, Nb, V and Ta, grown on both single- and polycrystalline substrates have shown an enhanced hardness for a particular repeat period, usually in the range of 3-10 nm.